1. Field of Invention
The invention relates to a device for detecting an electric current for driving an electric motor and, particularly, to a device for detecting a motor drive current that is adapted to being used in a drive unit for electric cars and in a drive unit for hybrid cars.
2. Description of Related Art
An inverter for driving electric motors is required by AC electric motors (in this specification, a motor and a generator, that also works as a motor, are both called electric motors) that are driven by a DC power source, e.g., a storage battery, in a drive unit for electric cars and in a drive unit for hybrid cars. The inverter is controlled based upon control signals which a motor control unit sends to a signal line of the inverter. During power running, a DC current supplied from the battery through a DC power line is converted into currents of the U-, V- and W-phases, and these currents are fed to three-phase coils of the electric motor through three-phase AC power lines. At the time of generation or regeneration, further, the currents of the U-, V- and W-phases generated in the three-phase coils of the electric motor are fed through the three-phase AC power lines, are converted into DC currents, and are supplied to the battery through the DC power line.
A current sensor for detecting a drive current is provided for the three-phase AC power lines that connect the electric motor to the inverter in order to monitor a state where the motor is driven by the inverter and to send data to the motor control unit. FIG. 12 concretely illustrates a conventional manner for arranging the current sensors for the three-phase AC power lines. In the arrangement as shown, a printed board d of a current sensor is arranged near the extended positions a, b and c of three bus bars with the plate surface in parallel with the bus bars. In this arrangement, the printed board d is placed to be in parallel with a plane passing through the axes of the bus bars. If the direction in which the bus bars extend is denoted by the Z-axis, the direction in which the bus bars are arranged by the X-axis, and the direction at right angles therewith by the Y-axis, then, it becomes difficult to decrease the size in the direction of the Y-axis and to decrease the area for arranging the current detector in a direction traversing the bus bars.
In another manner of arranging the current detector, a printed board constituting the current detector is arranged at a portion of current feeder lines (bus bars) (see JP-A-9-304447, page 8, FIG. 4). In this example, the printed board is arranged neighboring the cores that are arranged so as to surround the peripheral surfaces of the plate-like bus bars. The bus bars penetrate through the printed board.